Instruction manual

FMLP1 • 7
The previous paragraphs have help to shed a little light on the basic
workings of a lowpass filter. To this point the reason for the grouped elements
of ‘L1 & C3’ and ‘L3 & C8’ were not specifically mentioned. These components
still follow the guidelines set forth earlier but their configuration yields an
almost magical result. Our main concern for using a lowpass filter is to limit
the harmful presence of any harmonic frequencies, especially those in the 2
nd
harmonic (176 - 216 MHz) realm. Using the formulas given prior for X
C
and X
L
,
you can see that the ‘tank’ circuit formed by the parallel LC components has a
certain frequency when both reactive elements reach the same impedance
(AC resistance in ohms). This point is called the resonant frequency (f
r
) and is
reached when X
C
= X
L
. If a signal at the resonant frequency is applied to the
input of the ‘tank’ circuit it will be greatly attenuated, so much so that it is
almost completely stopped from passing through. This component
configuration of a parallel LC network is often called a ‘Band Stop’ because of
its notch frequency response. If the component values are chosen such that f
r
is at the harmonic frequency we want to stop, then the standard lowpass filters
ability to reduce this signal is greatly improved.
The FMLP1 uses two of these ‘Band Stop’ elements, ‘L1 & C3’ and
‘L3 & C8’. By tuning coils L1 and L3, f
r
can be set at the harmonic frequencies
we want to reduce. As you will see in the construction phase, no special
equipment like an RF network analyzer or other expensive equipment will be
needed! The harmonic values for the FM band are so close (176 - 216 MHz)
that normal building variances will be more than adequate to filter them out.
Hopefully this brief explanation has given you new respect for what
can be done with a few carefully chosen components! Now comes the fun part
of building and using your FMLP1!
SURFACE MOUNT COMPONENT SOLDERING INSTRUCTIONS:
You’ll notice that the circuit board does not contain any holes for
component leads to pass through. This is because the SMT components will
be affixed to the “solder” side of the PC board, the side that contains the PC
traces. Be aware that the component view for assembly is looking at the
solder side of the PC board.
Patience is the key when installing surface mount components.
Typically, the first step (after identifying the component) is to “tin” one of the
PC traces that will connect to the part. Once this is accomplished, the part can
be installed by holding it with tweezers in contact with the “tinned” trace and
re-heating the solder (see the associated diagrams on the following page).
Another commonly used technique is to glue the surface mount